1. Field of the Invention
The invention generally relates to a method of transmitting stationary pictures, via a transmission medium having a very limited capacity, to a user for display of said pictures on the display screen of a monitor. The invention more particularly relates to a method of transmitting such pictures via a digital transmission medium, such as a compact disc or the like.
2. Description of the Prior Art
As is generally known, the compact disc is a digital storage medium which can store a given number of bits. The use of the compact disc as a transmission medium for 16 bit PMC encoded audio signals has been known for some years. A second use for the compact disc was in data ROM applications for data processing equipment. A third use is now being developed, namely as a transmission medium for digitalised video signals of quasi-stationary pictures which are accompanied by digitalised audio and possibly computer data.
In order to be able to transmit such a quasi-stationary picture via this transmission medium, the picture is divided into a number of lines (for example 280) and each line is in turn divided into a number of contiguous picture elements (for example, 384). Each picture element is assumed to have a uniform luminance and a uniform colour. A picture element, or "pixel," is fully defined by a number of picture characteristics or components; namely the luminance component Y(i, k) and the two colour difference components U(i, k) and V(i, k) or, which is the same, the three primary colour components R(i, k), G(i, k) and B(i, k). In this case i indicates the ordinal number of the line and k indicates the ordinal number of the picture element on this line (column).
Each picture component of a picture element is subjected to an encoding operation so that a uniform information word is obtained for each picture element, which will hereinafter be referred to as a video word. When PCM encoding is used, the picture components of each picture element are separately encoded; that is to say, independent of corresponding picture components of other picture elements. However, when the picture components are subjected to a DPCM encoding, basically only the difference between the corresponding picture components of each pair of two consecutive picture elements is subjected to a PCM encoding. Since the encoding of such a difference generally requires fewer bits than the encoding of each of the separate picture components, 4-bit DPCM encoding is adequate for each of the picture components for the envisaged novel use of the compact disc. This means that each picture element is characterized by a 12-bit video word.
To utilize the capacity of the transmission medium as economically as possible, the entire block of 280.times.384 video words is transmitted only once. The monitor has a picture memory which has at least as many memory locations as the number of picture elements in the picture. A picture element is uniquely associated with each memory location and such a memory location stores the transmitted video word of the relevant picture element. This picture memory is repeatedly read in the conventional manner in order to obtain the relevant picture on the display.
It is to be noted that it has been found to be unnecessary in practice to transmit both the luminance component and the two colour difference components of each picture element. It is sufficient to transmit the luminance component of each picture element and to transmit alternately the colour difference component U and the colour difference component V of successive picture elements. This means that each picture element can be characterized by an 8-bit video word. More particularly, the first four bits of this video word represent the luminance component of the picture element in DPCM format and the other four bits represent one of the two (alternating) colour difference components, likewise in DPCM format. The assembly of video words to be transmitted together defining a given stationary picture will hereinafter be referred to as a video block.
In the now envisaged novel use of the compact disc, a video block is accommodated in packets for transmission, and likewise for the other digitalised information constituents, sound and computer data. Each packet comprises, for example, 2352 bytes and each packet is roughly divided into two fields, namely into a packet header consisting of 24 bytes and a data field of 2328 bytes. The last 280 bytes in the data field may be utilized for fault protection and correction of the other bytes in the data field. They are used, for example, when the packet comprises computer data.
The packet header comprises, inter alia, synchronisation bytes for byte and bit synchronisation but also indicates whether the data in the data field represent picture information or audio or computer data. In these cases they are referred to as a video packet, an audio packet, and a computer data packet, respectively. The data field of an audio packet comprises 1164 audio words of 16 bits each, while the data field of a video packet comprises 2328 video words of 8 bits each.